Porous filtration membranes that are used for ultrafiltration, microfiltration, reverse osmosis, or the like, have recently find applications in various industries, such as drinking-water production, water and sewerage treatment processes, and waste liquid treatment.
Such porous filtration membranes are widely used as ultrafiltration or microfiltration membranes in, for example, water purification, and often made mainly of polyvinylidene fluoride, which has excellent water permeability, and mechanical and chemical resistance. The porous filtration membranes mainly made of polyvinylidene fluoride are, however, highly hydrophobic and prone to fouling.
Fouling is a phenomenon that is caused by deposit, on a porous filtration membrane, of causative substances present in the raw water, called foulant, such as poorly-soluble components, high-molecular solutes including proteins or polysaccharides, colloid, micro-solid, or microorganisms, which decreases permeation flux, and known as a leading cause of degradation of membrane performance.
Fouling may be coped with by washing the porous filtration membrane with periodical flow of a surfactant solution, or flow of water in reverse to the ordinary flow, i.e., backwashing, to remove the foulant. Also discussed is use of a pretreatment agent for fouling inhibition, or imparting of a fouling reducing effect to a filtration membrane per se by additional processing during production of the filtration membrane.
These measures have certain effects in fouling inhibition, but may not be sufficiently effective against fouling with proteins or microorganisms. Use of a pretreatment agent is required to be continuous, whereas a filtration membrane imparted with a fouling reducing effect cannot be used in a water-treatment facility.
As a relatively effective means against fouling with proteins or microorganisms, hydrophilization of membrane surface is known. For example, Patent Publication 1 discloses coating of a porous filtration membrane by soaking in a solution containing polyvinyl alcohol. This method, however, results in poor chemical resistance, i.e., sufficient fouling inhibition is not achieved, for example, after chemical wash with alkali, such as sodium hypochlorite.
Patent Publication 2 discloses coating of a porous filtration membrane made of polyvinylidene fluoride with a polymer of 2-methacryloyloxyethyl phosphorylcholine (sometimes abbreviated as MPC hereinbelow) and n-butylmethacrylate (sometimes abbreviated as BMA hereinbelow) by soaking the membrane in a solution containing the polymer. Patent Publication 3 discloses a fouling inhibitor composed of a polymer prepared from MPC and BMA, for a water-treatment filtration membrane.
These water-treatment filtration membranes processed with an MPC-BMA copolymer exhibit excellent fouling inhibition against model contaminated water prepared using proteins, such as papain or bovine serum albumin, as a contaminant, but may be fouled rapidly with and may not exhibit substantial inhibition against actual contaminated water.
Patent Publication 1: JPS 54-17978-A
Patent Publication 2: JP 2012-55870-A
Patent Publication 3: JP 2006-239636-A